Conventionally, actuators such as servomotors, linear motors, stepping motors, electromagnetic actuators, fluid pressure actuators, etc. have been used in various fields of for example industrial robots, precision machines and switching operation elements. Recently, there is an enhanced demand for miniaturized, lightweight and highly flexible actuators. As such actuators, polymeric actuators driven by an electric field are drawing attention. Demand for the polymeric actuators is especially growing in such fields of for example medical equipments, micro machines, industrial robots and personal robots.
As such polymeric actuators, actuators using polymeric dielectric materials which have an excellent output/mass ratio or output/volume ratio are disclosed for example in Patent Documents of Japanese Unexamined Patent Application Nos. 2003-506858, 2003-526213 and 2005-1885 as well as Japanese Patent No. 2698716. In these polymeric actuators, materials such as an acrylic elastomer, a silicone elastomer, a polyurethane elastomer, a polyvinyl alcohol-based gel and a polyvinyl chloride-based gel are used for the dielectric materials. When voltage of more than several hundreds to one thousand volts is applied, the materials are highly expanded in the film surface direction thereof. Though various theories have been proposed about this phenomenon of the polymeric actuators, Maxwell stress arisen from the electrostatic force generated by the applied electric field is thought to be a main driving source.
Maxwell stress is a force proportional to a dielectric constant or more accurately to the product of relative permittivity and the permittivity of vacuum, or to the square of the applied voltage. Therefore, in order to make the polymeric actuator demonstrate more powerful output force even when the same voltage is applied, the dielectric material is desired to have a higher dielectric constant. Furthermore, the amount of displacement (or stroke), which is one of the important indicators in evaluating the performance of polymeric actuators, becomes large as the Young's modulus becomes small, when there is no difference in the levels of the generated Maxwell stress. Accordingly, it is desirable that the dielectric material has a low Young's modulus. Dielectric materials disclosed in the above-mentioned Patent Documents meet these requirements. However, in the case of using the acrylic elastomer or the silicone elastomer, crosslinking is required for use as a dielectric material. Therefore, there are some difficulties in molding them into a desired shape. In addition, chemical crosslinking is necessary to keep these dielectric materials into a desired shape of, for example, a membrane-shape. Therefore, the molding process is somewhat cumbersome. The chemical crosslinking in such elastomers is not always homogeneous, so that stress concentration at molecular chain level occurs within the elastomers when deformed, causing poor mechanical strength. The polyurethane elastomer in general has high Young's modulus and poor weather resistance, being undesirable. In the polyvinyl alcohol-based gel and the polyvinyl chloride-based gel, a large amount of plasticizer having a high dielectric constant can be used, so that dielectric materials having a high dielectric constant and low Young's modulus can easily be prepared, but there are some problems such as changes in properties including deterioration of dielectric materials due to bleeding out of the plasticizer itself or migration of the plasticizer to neighboring members.
On the other hand, in Japanese Unexamined Patent Application No. 2003-526213 and Japanese Patent No. 2698716, thermoplastic elastomers such as styrene-butadiene block copolymer, etc. are disclosed as a dielectric material. However, the styrene-butadiene block copolymer has a poor relative permittivity of 2.2, so it is hard to say that the polymeric actuators made from this dielectric material has excellent performance.